Not sure about that. Setting BT to miniUART should allow foth BT and GPIO at the same time:
/boot/config.txt:
dtoverlay=miniuart-bt
force_turbo=1
The 1st line switches BT to the miniUART and the 2nd line is needed with miniUART BT to set the VPU core clock to a fixed frequency, this is required based on https://www.raspberrypi.org/documentation/configuration/uart.md / UARTs and Device Tree -section, 3rd chapter.